Mixing between a reference signal and a data signal is often necessary to extract information about an optical device or network. A probe signal and a reference signal originating from the same source are typically mixed, resulting in fringes that can be detected and used to assess information about the device being probed. In interferometric sensing, a reference signal is mixed with a signal whose phase and/or amplitude is modified by a parameter to be measured. The mixing produces an interference signal, and the amplitude of the interference signal depends on how efficiently the two optical signals mix.
Optical Time-Domain Reflectometry (OTDR) is a widely used tool for identifying problems in large optical networks. OTDR instruments provide measurements of the level of scatter present in a section of fiber, or at a discrete interface over long distances. Optical Frequency Domain Reflectometry (OFDR) may be used to provide data similar to that provided by OTDR over shorter ranges (tens of meters for OFDR instead of 1000's of meters for OTDR) and higher resolutions (tens of microns for OFDR instead of tenths of meters for OTDR). This change in distance scale allows OFDR to be used in applications where the dimensions of interest are centimeters instead of meters such as when optical coupler and switch networks are constructed. For example, OFDR may be used in module-level and sub-module-level diagnostics. The inventors discovered that the ability of OFDR to measure the complex spectral reflectivity of Rayleigh backscatter as a function of fiber length yields surprising new and very useful results, as will be described later.
Scatter is the process of redirecting the propagation of light. In an optical fiber, this occurs when light encounters a change in the geometry of the fiber core, or a change in the local index of refraction of a fiber. Scatter generally occurs at any interface such as connectors, poor splices, collimating optics, etc. Typically, light scattered from the forward propagating direction into the backward propagating direction is of primary concern and is called a reflection. Rayleigh scatter, in the context of optical fiber, describes the light scattered in the fiber due to the random nature of the glass structure in and around the fiber core. Although Rayleigh scatter is random in nature, it is fixed because the random pattern of the glass structure is “frozen” into the fiber. Loss is the removal of light from the intended forward propagating mode. Scatter is a form of loss, as is bend radiation and molecular absorption.
Scattered light may be measured and characterized using OFDR. A highly monochromatic beam of light is injected into the optical system or device to be tested. The wavelength/frequency of that light is varied slowly with a time-linear sweep, and the optical signal back-scattered from the optical system is detected by coherently mixing the back-scattered signal with the reference input signal. The beat frequency component of the mixed signal, (corresponding to an interference signal), is measured to determine a position of the back-scattering (reflection) point in the optical system/fiber. The interference signal amplitude also determines a back-scattering factor and an attenuation factor for the reflected light.
When couplers are used in an optical network, reflectometric interrogation from one side of a coupler produces a measurement in which the backscatter from the two output legs of the coupler is combined into a single trace as illustrated in FIG. 1. Although scattering events and losses can be identified, one cannot determine from simple OFDR or OTDR measurements in which fiber a specific loss event occurred. Without the ability to distinguish different branches of a network, it is possible to identify that there is a faulty optical component in an optical network that contains multiple fibers, but nonetheless cut the wrong fiber when trying to replace the faulty component. In this and other types of cases, knowledge of which individual fiber caused a loss event (rather than the general information that one of the fibers caused a loss event) would be very helpful in achieving a quick and efficient repair of the system.
The inventors determined ways to use complex data obtained from OFDR measurements of backscatter for an optical device under test (DLT). A fiber segment DUT can be identified by itself, within a longer fiber DUT, or within an optical network DUT that includes multiple fibers coupled to perform one or more functions. In other example applications, OFDR backscatter data can be used to identify where in a DUT (and for a DUT with plural fibers, in which fiber) a loss occurred and to identify where in a DUT (and for a DUT with plural fibers, in which fiber) a change occurred (e.g., a temperature change resulting in a change in fiber length). These and other advantageous applications are achieved using previously-determined, complex OFDR measurements for fiber segments, with each fiber segment having its own associated “scatter pattern.” In one example embodiment, a scatter pattern associated with the fiber segment is obtained and stored in memory. Optical Frequency Domain Reflectometry (OFDR) is used to obtain the scatter pattern. The scatter pattern may be, for example, a Rayleigh scatter pattern. The scatter pattern may then be used to identify the fiber segment.
Another example application uses the scatter pattern to locate a fiber segment in a device under test (DUT). The fiber segment scatter pattern corresponds to a first set of complex reflectivity numbers which are a function of frequency. OFDR is used to process the DUT and generate a second set of complex reflectivity numbers. A comparison is performed using the first and second sets of complex reflectivity numbers. The location of the fiber segment in the DUT is determined based on the comparison.
In one detailed example implementation, a first amplitude of the first set of complex reflectivity numbers is calculated and the mean is removed. A second amplitude of the second set of complex reflectivity numbers is calculated, and the mean is removed. The first and second amplitude signals are then cross-correlated, and the maximum cross-correlation is identified. The maximum cross-correlation corresponds to the location of the fiber segment in the DUT.
Other non-limiting example applications relate to calculating a loss associated with the fiber segment or a change in fiber length if there has been some kind of changed situation or condition. The first set of complex reflectivity numbers is shifted by an amount corresponding to the determined location, and a complex conjugate of the shifted first set of complex reflectivity numbers is calculated. The complex conjugate is multiplied by the second set of complex reflectivity numbers to generate a complex product. The complex product is averaged over a distance associated with the DUT. A change in amplitude of the averaged complex product is determined. The change typically corresponds to a loss associated with the fiber segment in the DUT as a function of distance along the DUT. Alternatively, a change in phase for the averaged complex product may be determined. The phase change corresponds to a temperature change that affects a fiber length associated with the DUT.